In induction plasma torches, a strong oscillating magnetic field is generated by an induction coil and applied to a gas passing through this coil to ionise the gas and form a plasma. Such induction plasma torches use the concept of inductive coupling itself consisting of inductively coupling a radio frequency (RF) field to the flowing gas. The inductive coupling heats the gas to a high temperature, typically 9 000° C. At that temperature, the gas turns into a plasma of positively charged ions and electrons. Plasma torches are typically used for spectroscopic elemental analysis, treatment of fine powders, melting of materials, chemical synthesis, waste destruction and the like. These applications derive from the high temperatures inherently associated with plasmas.
Early attempts to produce plasma by induction involved the use of a single-coil high frequency RF field (in the megahertz range). Attempts were also made to induce plasma formation using a lower frequency RF field (under 400 kHz) but were unsuccessful. These attempts to form plasmas using lower frequencies were driven by the belief that, at lower frequencies, the plasma is larger and has a more uniform temperature. It was also recognised at this stage that the process of igniting the plasma was different from that of running the plasma once ignited.
When operated at a high power level (above 10 kW) and a pressure equal to or higher than one (1) atmosphere, industrial inductive torches are difficult to ignite and to run stably. A dual coil, or RF-RF hybrid design has been proposed as a method to alleviate some of these problems.
Experimentation involving the use of dual coil induction plasma torches was underway in the mid 1960s. The article by I. J. Floyd and J. C. Lewis, “Radio-frequency induced gas plasma at 250-300 kc/s”, Nature, Vol. 211, No. 5051, at p. 841 discloses the use of a dual coil system including:                a higher frequency coil operating in the megahertz range to ignite, or initiate the plasma; and        a second “work” coil operated at a lower frequency.Continuing work on the dual coil plasma torch also revealed that, as expected, the lower frequency coil produced a plasma with a much more homogenous temperature. This, combined with a reduction of axial pressure, brought about an increase in dwell time and penetration of products which gave rise to benefits in the form of improved conditions for spheroidization treatment, or the spraying of powders.        
Additionally, the presence of two separate induction stages was found to allow hot gases exiting the first stage to be mixed with a different gas which would otherwise adversely affect plasma sustainability. Moreover, the cascading of two induction coils allows the working parameters of the torch to be optimised, thereby increasing efficiency and reducing the power required to operate the plasma torch.
Two types of power supply have been used for supplying the considerable amount of power required to operate an induction plasma torch: a tube-type oscillator power supply and a solid state power supply.
Tube-type oscillator power supplies are notoriously inefficient with typically 40% of the input power being lost in the oscillator and tank circuit and only 20 to 40% of the input power being available as plasma enthalpy in the hot gas.
Solid state power supplies provide for more efficient operation and, therefore, constitute a better alternative. They exhibit, in comparison to tube-type oscillator power supplies, an overall efficiency in converting electrical energy from a relatively low supply voltage of 440 or 560 Volts at 50 or 60 Hz to a higher voltage of 1 500 to 3 000 Volts at 300 to 400 kHz. This increase in efficiency is largely due to the replacement of the standard, water-cooled triode or pentode tube oscillator with a solid state transistorised circuit.
Solid state power supplies, however, currently have a characteristic low frequency range of operation (typically between 300 to 400 kHz) and therefore are generally unsuitable for producing the required RF signal to the high frequency coil which is used to inductively ignite the plasma. Additionally, the use of efficient solid state power supplies has been proscribed in the applications requiring the ignition and operation of a plasma torch under atmospheric pressure or soft vacuum conditions.
Furthermore, existing dual coil designs using tube-type oscillator power supplies result in serious interactions between the control circuits of the two power supplies which can only be resolved by imposing a minimum separation between the coils. The imposition of a separation between the coils seriously affects the uniformity of the temperature field in the resulting plasma and has a direct impact on efficiency.